The backlight or front light having a light-emitting diode (LED), as a light source, has the features of size-reduced light source, long life, high performance, unnecessary exclusive power source and so on, and in many cases is used as a light of a liquid crystal display unit used in a display of a portable apparatus. Of among these features, two points, i.e. high efficiency and small size, are particularly important. This is because the surface light source device, if high in efficiency increases the brightness on a light-emitting surface of the surface light source device, making easy to view an image on the liquid crystal display unit. Furthermore, with high efficiency, the surface light source device is decreased in consumption power and increased in battery life. Meanwhile, the portable apparatus cannot be size-reduced without the size reduction of the surface light source device. This requires reducing the area of a non-emitting region and the thickness as well.
Also, in the case of using as a display of a portable apparatus, it is satisfactory to view the display at a front thereof. There is less need to view in an oblique direction. In many cases, it is rather preferred not to be seen obliquely. Accordingly, abroad directivity characteristic is not required for the light to be emitted from the surface light source device. In order to improve the efficiency of the surface light source device, it is preferred to emit light only in a direction having a certain degree of spread, in a direction of a normal line given on a light emitting surface of the surface light source device.
Now, in FIGS. 1 and 2 is shown an explosive perspective view and sectional view of a surface light source device of a general structure. The surface light source device 1, for use as a backlight, is structured with a light conductor plate 2 for light confinement, a light emitting part 3 and a reflection plate 4. The light conductor plate 2 is formed of a transparent resin having a high refractive index, such as a polycarbonate resin or a methacrylic resin. The light conductor plate 2 has, in a lower surface, diffusion patterns 5 formed by concavo-convex working, dot printing with a diffuse-reflection ink or the like. The light emitting part 3 has a plurality of light-emitting diodes 7 mounted on a circuit board 6, and opposed to a side surface (light incident surface 2a) of the light conductor plate 2. The reflection plate 4, formed by a white-resin sheet, is bonded at both sides onto a lower surface of the light conductor plate 2 by double-sided tapes 8.
In the surface light source device 1 like this, the light, emitted from the light emitting part 3 and guided at a light incident surface 2a into the light conductor plate 2, travels while repeating regular reflections at between the upper surface (light emitting surface 2b) and the lower surface of the light conductor plate 2 as shown in FIG. 2. Upon incidence on the diffusion pattern 5, diffuse-reflection occurs. In case the incidence is at an angle smaller than a critical angle of total reflection toward the light emitting surface 2b, the light is emitted at the light emitting surface 2b to the outside. Meanwhile, the light, passed a point where there is no diffusion pattern 5 on the lower surface of the light conductor plate 2, is reflected by the reflection plate 4 to return again into the light conductor plate 2, preventing light amount loss at the lower surface of the light conductor plate 2.
However, although the surface light source device 1 having such a structure is simple in structure, it is structurally worse in light utilization efficiency. It has been impossible to emit only approximately 20% of the emission light of from the light-emitting diode 7, at the light emitting surface 2b of the light conductor plate 2.
Furthermore, in the surface light source device 1 in such a structure, in order to direct the light emitted in a direction approximate to the parallel with the light emitting surface 2b of the light conductor plate 2 toward a direction perpendicular to the light emitting surface 2b, a diffusion plate 9 is used superposed on the light conductor plate 2 as shown in FIG. 3. The surface light source device 1 has an increased thickness, making it difficult to size-reduce the surface light source device 1. Moreover, where using a diffusion plate 9, the light passed the diffusion plate 9 turns into a Lambertian light. Because of broad directivity characteristic, the brightness at the front is low, thus lowering the utilization efficiency of light.
Meanwhile, the surface light source device 1 structured as shown in FIG. 1 uses a light emitting part 3 mounted with a plurality of light-emitting diodes 7. Thus, it is difficult to size-reduce the light emitting part 3. Also, the power consumption of the surface light source device 1 cannot be reduced.
On the other hand, the surface light source device using light-emitting diodes is used on the commodities having a strong requirement of portability, such as cellular phones and PDAs, because of its reduced size and weight. There is a strong need for the life increase of the power source in view of improved portability. The reduction of power consumption is strongly desired for the surface light source device for use in the same. For this reason, there is advancement of decreasing the number of light-emitting diodes to be used.
Under this situation, there is a proposal of a surface light source device 11 as in FIG. 4 using one light-emitting diode (JP-A-11-231320). In this surface light source device 11, a light emitting part 12 is made by arranging a light-emitting diode 14 opposed to an end of a wedge-formed bar member 13, to arrange the light emitting part 12 opposed to a light incident surface 15a of a light conductor plate 15. The light conductor plate 15 also is in a wedge form to have, on its lower surface, a diffuse-reflection sheet 17 to diffuse-reflect the leak light at the lower surface of the light conductor plate 15 and return it into the light conductor plate 15. Opposed to a light emitting surface 15b of the light conductor plate 15, superposed are a diffusion plate 18 and the prism sheet 19. Also, a prism-formed pattern 16 is formed on the light incident surface 15a of the light conductor plate 15.
According to the surface light source device 11 like this, the light utilization efficiency of a light source is improved because of the capability of driving with one light-emitting diode 14. However, because light diffusion is done by the diffuse-reflection sheet 17 and diffusion plate 18, directivity is broadened as shown in FIG. 5. Due to the lower in directivity characteristic, the efficiency is not sufficient as the entire of the surface light source device 11. Furthermore, because the diffusion plate 18 and prism sheet 19 are superposed on the light conductor plate 15, the surface light source device 1 is increased in thickness. The light source 12 is size-increased by the bar-formed member 13. Thus, there has been difficulty in reducing the size.
Also, there is a showing in FIG. 6 as another surface light source device 21 using one light-emitting diode. This has one light-emitting diode 23 arranged opposed to a center of a light incident surface 22a of the light conductor plate 22, to concentrically arrange U-shaped diffusion patterns 24 about the light-emitting diode 23 in a lower surface of the light conductor plate 22. Each diffusion pattern 24 extends in a direction orthogonal to a direction connected to the light-emitting diode 23.
Then, in this surface light source device 21, the light emitted from the light-emitting diode 23 enters at the light incident surface 22a the light conductor plate 22 to travel within the light conductor plate 22. Within the light conductor plate 22, the light striking the diffusion pattern 24 is reflected upon an interface of the diffusion pattern 24 as shown in FIG. 7 and emitted toward the light emitting surface 22b in a surface of the light emitting surface 22b. Only the light incident, at an incident angle smaller than a critical angle of total reflection, on the light emitting surface 22b is allowed to emit at the light emitting surface 22b to the outside.
However, the diffusion pattern 24 is circumferentially uniform about the light source. The light if striking the diffusion pattern 24, in plan view, is not changed in traveling direction. There is no diffusion action of light in the circumferential direction about the light source. Consequently, the evenness of light in the circumferential direction is determined depending solely on the circumferential distribution of light amount of the light source or the distribution of amount of the light entered at the light incident surface into the light conductor plate.
As a result of this, the circumferential directivity characteristic is narrow in this surface light source device 21, the directivity of the light emitted from the light conductor plate 22 is extremely narrow in a widthwise direction of the light conductor plate 22 but broad in a lengthwise direction as shown in FIG. 8. For this reason, a diffusion plate for moderating the widthwise directivity is essentially required, increasing the thickness of the surface light source device 21. Meanwhile, because the diffusion plate causes diffusion not only in the widthwise direction but also in the lengthwise direction, the lengthwise directivity is extremely worsened and hence the brightness in the perpendicular direction is lowered.
Next, in FIG. 9 is shown a surface light source device 31 for use as a front light. In this surface light source device 31, a plurality of slits 33 are formed of a material different from the light conductor plate 32 or air on an upper surface of the light conductor plate 32. A linear light source 34 just like a cold-cathode tube is arranged opposed to a side surface of the light conductor plate 32. Consequently, the light, exited from the linear light source 34 to be incident on the light conductor plate 32, is totally reflected upon the slits 33 and emits at a lower surface of the light conductor plate 32. This is returned to the light conductor plate 32 by the reflection upon a reflective-type liquid crystal display panel 35, to pass through between the slits 33 and emit at the upper surface of the light conductor plate 32.
However, in the surface light source device 31 like this, because sufficiently many slits 33 cannot be provided, the utilization efficiency of light is low. Also, because the slits 33 must be formed in the interior of the light conductor plate 32, there has been a difficulty of manufacture.
Similarly, there is a proposal of a surface light source device for front light having a plurality of island regions formed different in refractivity index from the other within the light conductor plate (JP-A-7-199184). This is also insufficient in light utilization efficiency, and the manufacture is difficult.
Meanwhile, the other surf ace light source devices for front light include the use of a light conductor plate gradually, stepwise reduced in thickness as distant from the light source (JP-A-10-326515), the provision of a prism sheet on a lower surface of a wedge-formed conductor plate (JP-A-10-301109), the forming with a concavo-convex pattern sectionally in rectangular in a lower surface of a light conductor plate (JP-A-10-123518), and the provision with V-formed groove strips in an upper surface of a wedge-formed conductor plate (JP-A-11-64641).
In the surface light source device like this, despite having a relatively simplified structure, the utilization efficiency of light is still insufficient thus darkening the screen.